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The Solar System
As far as we know, there are nine planets locked in orbit around the Sun. Only one, our own
Earth, supports life. But there are countless other suns throughout countless galaxies
scattered across the expanse of the universe. We still don’t know if life exists on another
planet in some other galaxy.
But we do know more and more all the time about our own solar system. During the past 15
years, space probes such as the Mariner and Voyager missions have given us tremendous
detail about all the planets in this system. Tiny by comparison to the Milky Way, our solar
system is awesome, nevertheless, when considered in human scale.
Mercury
Named for the wing-footed messenger of the Roman gods, Mercury races around its orbit at
a dizzying speed of 30 miles (48 kilometers) per second, making the Mercurial year only 88
Earth days long. In contrast, one rotation around its axis—or a single day—takes almost 59
Earth days.
Geology
Mariner 10 gave us a wealth of information about Mercury when it approached the planet in
1974 and 1975. Because Mercury has no water and barely any atmosphere, no erosion has
taken place on its surface. We see Mercury much the way it was soon after it formed.
Core
We learned that Mercury has an extremely weak magnetic field, which could indicate a hot
metallic core, such as molten iron. Geologists think Mercury may be the most iron-rich
planet in the solar system.
Crust
Mercury’s crust seems to be silicate, like that of Earth.
Craters
The planet’s surface, viewed for the first time via Mariner’s cameras, is covered by craters.
The battering occurred during the early period of the solar system when clumps of material
were ramming into each other to form the planets.
Atmosphere
Mercury doesn’t have what we think of as an atmosphere—that is, a gaseous envelope that
produces clouds and weather or protects the surface of the planet from some of the harmful
solar radiation. The weak magnetic field on Mercury captures only the barest perceptible
trace of charged particles from the Sun.
Venus
Shrouded in the cloak of mystery, Venus, our nearest planetary neighbor, takes the name of
the Roman goddess of love. For some unknown reason, Venus rotates on its axis in
retrograde—that is, in the reverse direction of its revolution around the Sun.
Geology
Geologically, Venus appears to have some similarities to Earth. Its crust is probably granite,
overlying a basaltic mantle and a iron-nickel core. The geologic activity that we are familiar
with on Earth seems not to exist on Venus, except for the presence two volcanoes along a
fault line.
Atmosphere
Venus’s veil of mystery is really an impenetrable, heavy layer of sulfuric acid clouds above
an atmosphere consisting of about 96 percent carbon dioxide.
Greenhouse effect
Sunlight entering Venus’s atmosphere is converted to heat radiation and is prevented from
escaping by carbon dioxide—a phenomenon we call the “greenhouse effect.” Surface
temperatures reach 800 degrees Fahrenheit (341.33 degrees Celsius), and the atmosphere
appears to be constantly flashing with lightning.
Weight of atmosphere
For an idea of the weight of Venus’s atmosphere, consider that walking on the surface of
Venus would be comparable to walking on the ocean floor at a depth of half a mile.
Earth
For about 500 million years after its initial formation, the Earth remained at a rather stable
2000 degrees Fahrenheit (874.68 degrees Celsius). Comprised predominantly of iron and
silicates, the Earth also contained small amounts of radioactive elements, mostly uranium,
thorium, and potassium. As these elements decayed, they produced radiation that gradually
heated the Earth, melting the iron and silicates. The iron sank to the center, forcing the
lighter silicates to the surface and causing the violent processes that formed the Earth’s
surface as we know it and that continue to form it even today.
Geology
Between the iron core and the solid rock crust lies a mantle of heavy silicate rock that is
1800 miles thick (2880 kilometers). The mantle is neither solid nor liquid, but has a viscous,
yielding consistency upon which the crust actually floats. The crust is not a single, solid
mass, but rather a collection of separate plates, along whose boundaries the geologic
processes continue to work on the planet’s surface.
Atmosphere
The Earth’s atmosphere extends more than 400 miles (640 kilometers) into space. From
sea level to a height of about 60 miles, the atmosphere is 78 percent nitrogen, 21 percent
oxygen, and the remaining 1 percent a combination of argon, carbon dioxide, neon, helium,
krypton, xenon, and tiny amounts of some other gases.
Moons
The Earth’s single moon is large in comparison with the natural satellites of all the other
planets except for Pluto. Astronomers actually consider the Earth-Moon system to be a
double planet. Its size gives the Moon a significant gravitational influence on the Earth,
causing our oceans to have tides.
Mars
Because it is so visible, and its atmosphere is so thin and transparent, we know more about
Mars than we do any other planet except Earth. The Martian day is roughly the same as an
Earth day, since Mars takes 24 hours and 37 minutes to rotate upon its axis, but its year is
687 Earth days long. Its distance from the Sun can vary by about 36 million miles because
of its eccentric orbit.
Geology
The blood-red color of Mars inspired the Romans to name the planet for their god of war,
but the color actually is caused by iron oxide rust on the planet’s surface. Canal-like
configurations on the surface of Mars have long inspired scientists to maintain the hope of
finding water, which might mean that life exists on the planet.
Atmosphere
Mars probably developed its atmosphere by spewing gases from its interior, as Earth did.
But Mars, with a mass barely more than 10 percent of Earth’s, had insufficient gravity to
retain the lighter gases. Today, the Martian atmosphere is mostly carbon dioxide.
Moons
Two tiny moons orbit Mars. They are tiny, irregularly shaped satellites with heavily cratered
surfaces. The larger of the two is called Phobos and the smaller is called Deimos. Phobos
is only about 14 miles (23 kilometers) across and Deimos is only about 8 miles (13
kilometers) across.
The Asteroid Belt
Based on the distances between the nine known planets, there should be a tenth planet
between Mars and Jupiter. Indeed, where the planet should be there is a band of asteroids,
some as large as 620 miles (1000 kilometers) across, some just tiny pebbles.
Jupiter
Jupiter is the largest of the planets in our solar system and is named for the king of all the
Roman gods. It is not considered a terrestrial planet in that its “surface” is a swirling mass of
liquid hydrogen with a core of iron about the size of the Earth.
Atmosphere
Besides hydrogen, the atmosphere contains helium, methane, and visible clouds of icy
ammonia that blow across the face of the planet. A spot at least twice the diameter of Earth
on Jupiter’s surface is probably a 13,000-mile-wide storm that has been raging for
thousands of years. The spot’s dark red color suggests that it may tower thousands of miles
above the lighter-colored clouds.
Moons
Jupiter has 16 moons, the largest of which are Europa, Ganymede, Io, and Callisto. Each of
these major moons has surface characteristics distinctly different from the others. When
Jupiter is nearest the Earth, the moons can be seen clearly through field glasses as they
move around the giant planet.
Saturn
Saturn is the second largest planet in the solar system, but probably the lightest because it
is the least dense. In fact, because the ratio of size to weight is so large, Saturn would
probably float in water, if a large enough body of water existed. Like Jupiter, Saturn is a ball
of gas and liquid, with a possible solid core about the size of Earth.
Atmosphere
Like Jupiter, Saturn’s atmosphere is torn by violent storms that rage endlessly. Some storm
systems are larger than Europe and Asia combined. The entire atmosphere is enveloped by
a hydrogen haze. Beneath the haze the atmosphere is mostly hydrogen and helium with
some methane and ammonia ice crystals.
Moons
Two Voyager probes observed Saturn in 1980 and 1981 and sent evidence back to Earth
that Saturn has between 21 and 23 moons, more than any other planet. One of Saturn’s
moons, Titan, is larger than Mercury and Pluto. All of the moons of Saturn are made of
water ice, some containing rock.
Rings
The most distinguishing feature of Saturn is, of course, its rings. They begin about 7000
miles (11,200 kilometers) from the “surface” of Saturn and extend, with gaps, to about
35,000 miles (56,000 kilometers). The rings are composed of dust and relatively small
particles of ice and are clearly visible from Earth through a small telescope.
Uranus
Most surprising of all the planets is Uranus, the seventh planet from the Sun. Uranus lies at
roughly a 90-degree angle to the Sun, so that one of its poles constantly points almost
directly at the Sun as it rotates. Oddly enough, the temperature at both poles is about the
same.
Atmosphere
Uranus is a nonterrestrial planet whose surface is a superheated ocean of water. A dense
atmosphere of mostly hydrogen and helium provides the pressure that both heats the water
to thousands of degrees and prevents it from boiling away. The high temperatures in return
prevent the pressure from solidifying the water.
Moons
Also surrounding Uranus are 15 moons. Until Voyager 2 began sending information back to
Earth about Uranus in January 1986, only five moons were known—Oberon, Titania,
Umbrial, Ariel, and Miranda.
Rings
Like Saturn, Uranus is surrounded by rings. This discovery was made by astronomers in
1977, when they observed an occultation, which is the eclipse of a star by a planet. They
noticed that the star’s light flickered for 35 minutes before the occultation by Uranus. Then,
after Uranus passed in front of the star, the light reversed its flickering before becoming fully
visible again.
Neptune
The giant blue planet inspired ancient peoples to name it after the god of the deep blue sea.
We know less about Neptune than we do about the other eight planets, although Voyager 2
gave us new information.
Pluto
Pluto, the ninth and, as far as we know, last planet in our solar system, was predicted by
Percival Lowell, who noticed that there were unexplained irregularities in the orbits of
Uranus and Neptune. In 1930, Clyde Tombaugh actually discovered Pluto exactly where
Lowell had predicted it would be. The name Pluto is in keeping with the tradition of Roman
gods and is appropriate—Pluto was the god of the underworld, and the planet is a dark and
frozen place—but it was chosen because the first two letters are Percival Lowell’s initials.